3 Answers
3

If you connect your phone via the USB it gets charged. It doesn't matter if you don't use the USB connection at all or transfer files or tether your data connection or do any other stuff while it's connected.

The charging circuit doesn't even know that it's connected via USB and that there's data flowing. It just happily charges the phone with the power supplied via USB.

Side note:
The battery (LiIon) degrades over time, if you use it or not. There are however differences in the speed of degradation, depending on usage, storage temperature and charging state (best is around 60-70% charged as far as I know). This is however the usual degradation which you asked not to factor in. And this degradation definitely doesn't change if you tether or not.

I think the phone does not actually charge even if seems to be charging. That is the reason they ask you to have more than 50% battery before you update via USB. Isn't it?
–
SidJan 6 '13 at 2:45

I disagree. Of course does it charge, USB provides 500mA (USB3 and some ports even more). That 50% limit has no reason to exist at all. My old xperia even runs without battery, including the ability to upgrade
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ce4Jan 6 '13 at 3:44

Thanks for the info I was just worried that the constant charge and discharge when tethering might degrade the battery other than anything usual
–
MysterioJan 6 '13 at 7:51

1

The charging circuit will stop charging and the phone will be powered over USB (if it doesn't draw more than 500mA), so the battery should ideally be idle as soon as it's charged (i.e. no current flow).
–
ce4Jan 6 '13 at 11:51

yeah it would certainly shorten your battery's life. Every battery has a maximum number of charge-discharge cycles. So when the phone is tethered via USB, battery is charging.
The more FREQUENTLY you charge your battery, the more you shorten its life. This applies not only to tethering/data transfer, but also to the habit of many of charging phones for a short duration, then again discharging it , then again charging it.
This drastically reduces the battery's lifespan.

The best habit is , Charge the phone to 100% . Then use it till its down to 15-20%. This is the best time when you should plug in your device. Mind that discharging to 0% also has adverse effects on backup of a battery.

So IMO Bluetooth tethering is the best solution for 3G speeds.. It is available since ICS . If you want to utilize full 4G speeds, prefer WiFi tethering. While tethering, when the phone's battery comes down below 20% , consider charging it. It's always handy to carry a spare battery with your Droid.

All batteries have some recharging cycle. If you repeatedly use USB that means you are charging repeatedly. This reduces battery life by consuming recharge cycle early.

And contrary to manufactures demand to have 50% battery for updating the software, you can updated it even with 5% if it lasts until your software update is complete. I've done it in my Sony W8. In Sony W8 I've updated in just 10% of battery, even though Sony is worst performer in battery.